This invention relates generally to the manufacture of high speed semiconductor microprocessors, application specific integrated circuits (ASICs), and other high speed integrated circuit devices. More particularly, this invention relates to advanced back-end-of-line (BEOL) metallization structures for semiconductor devices using low-k dielectric materials. The invention is specifically directed to an advanced BEOL interconnect structure having a low-k cap layer, and a method of forming the interconnect structure using a plasma-enhanced chemical vapor deposition (PE CVD) process to form the cap layer.
In semiconductor devices, aluminum and aluminum alloys have been used as the traditional interconnect metallurgies. While aluminum-based metallurgies have been the material of choice for use as metal interconnects over the past years, concern now exists as to whether aluminum will meet the demands required as circuit density and speeds for semiconductor devices increase. Because of these growing concerns, other materials have been investigated as possible replacements for aluminum-based metallurgies.
One highly advantageous material now being considered as a potential replacement for aluminum metallurgies is copper, because of its lower susceptibility to electromigration failure as compared to aluminum, as well as its lower resistivity.
Despite these advantages, copper suffers from an important disadvantage. Copper readily diffuses into the surrounding dielectric material during subsequent processing steps. To inhibit the diffusion of copper, copper interconnects are often capped with a protective barrier layer. One method of capping involves the use of a conductive barrier layer of tantalum, titanium or tungsten, in pure or alloy form, along the sidewalls and bottom of the copper interconnection. To cap the upper surface of the copper interconnection, a dielectric material such as silicon nitride is typically employed.
For example, state-of-the-art dual damascene interconnect structures comprising copper interconnects are described in xe2x80x9cA High Performance 0.13 xcexcm Copper BEOL Technology with Low-k Dielectric,xe2x80x9d by R. D. Goldblatt et al., Proceedings of the IEEE 2000 International Interconnect Technology Conference, pp. 261-263. A typical interconnect structure using low-k dielectric material and copper interconnects is shown in FIG. 1. The interconnect structure comprises a lower substrate 10 which may contain logic circuit elements such as transistors. A dielectric layer 12, commonly known as an inter-layer dielectric (ILD), overlies the substrate 10. In advanced interconnect structures, ILD layer 12 is preferably a low-k polymeric thermoset material such as SiLK(trademark) (an aromatic hydrocarbon thermosetting polymer available from The Dow Chemical Company). An adhesion promoter layer 11 may be disposed between the substrate 10 and ILD layer 12. A layer of silicon nitride 13 may be disposed on ILD layer 12. Silicon nitride layer 13 is commonly known as a hardmask layer or polish stop layer. At least one conductor 15 is embedded in ILD layer 12. Conductor 15 is typically copper in advanced interconnect structures, but may alternatively be aluminum or other conductive material. A diffusion barrier liner 14 may be disposed between ILD layer 12 and conductor 15. Diffusion barrier liner 14 is typically comprised of tantalum, titanium, tungsten or nitrides of these metals. The top surface of conductor 15 is made coplanar with the top surface of silicon nitride layer 13, usually by a chemical-mechanical polish (CMP) step. A cap layer 17, also typically of silicon nitride, is disposed on conductor 15 and silicon nitride layer 13. Silicon nitride cap layer 17 acts as a diffusion barrier to prevent diffusion of copper from conductor 15 into the surrounding dielectric material.
A first interconnect level is defined by adhesion promoter layer 11, ILD layer 12, silicon nitride layer 13, diffusion barrier liner 14, conductor 15, and cap layer 17 in the interconnect structure shown in FIG. 1. A second interconnect level, shown above the first interconnect level in FIG. 1, includes adhesion promoter layer 18, ILD layer 19, silicon nitride layer 20, diffusion barrier liner 21, conductor 22, and cap layer 24.
The first and second interconnect levels may be formed by conventional damascene processes. For example, formation of the second interconnect level begins with deposition of adhesion promoter layer 18. Next, the ILD material 19 is deposited onto adhesion promoter layer 18. If the ILD material is a low-k polymeric thermoset material such as SiLK(trademark), the ILD material is typically spin-applied, given a post apply hot bake to remove solvent, and cured at elevated temperature. Next, silicon nitride layer 20 is deposited on the ILD. Silicon nitride layer 20, also known as a hardmask layer or polish stop layer, is patterned by conventional photolithography techniques, and then acts as a mask during subsequent etching of ILD layer 19, adhesion promoter layer 18 and cap layer 17, to form at least one trench and via. The trenches and vias are typically lined with diffusion barrier liner 21. The trenches and vias are then filled with a metal such as copper to form conductor 22 in a conventional dual damascene process. Excess metal is removed by a CMP process. Silicon nitride layer 20 acts as a polish stop layer during the CMP process. Finally, silicon nitride cap layer 24 is deposited on copper conductor 22 and silicon nitride layer 20.
Silicon nitride layers 13 and 20 are not necessary components of the finished interconnect structure, because after planarization by CMP, dielectric cap layers 17 and 24 are deposited across copper conductors 15 and 22, and ILD layers 12 and 19. In fact, hardmask or polish stop layers 13 and 20 are often polished away completely in at least some portions of the wafer, prior to dielectric cap layer deposition.
Due to the need for low temperature processing after copper deposition, the cap layers 17 and 24 are typically deposited at temperatures below 450xc2x0 C. Accordingly, cap layer deposition is typically performed using plasma enhanced chemical vapor deposition (PE CVD) or high density plasma chemical vapor deposition (HDP CVD) wherein the deposition temperature generally ranges from about 200xc2x0 C. to about 500xc2x0 C.
PE CVD and HDP CVD silicon nitride have been used for many other applications in semiconductor device manufacturing. However, in using a silicon nitride cap for copper interconnects, conventional PE CVD silicon nitride creates reliability problems. In particular, silicon nitride films deposited using conventional PE CVD processes generally exhibit poor adhesion to the copper surface. For instance, some nitride films delaminate and form blisters over patterned copper lines, particularly during subsequent dielectric depositions, metallization, and chemical-mechanical polishing. After being deposited onto copper metallurgy, additional insulating layers generally will be deposited over the silicon nitride film. However, subsequent deposition of insulating layers onto the nitride film will produce stress which can cause the silicon nitride film to peel from the copper surface. This delamination results in several catastrophic failure mechanisms, including lifting intermetal dielectrics, lifting copper lines, and copper diffusion from uncapped copper lines. Such results are generally seen in dual damascene processing wherein delamination of the silicon nitride hardmask layer generally occurs during copper chemical-mechanical polishing (CMP).
Silicon nitride films deposited using HDP CVD generally exhibit superior adhesion to copper surfaces as compared to PE CVD silicon nitride films. However, HDP CVD silicon nitride films are more costly to produce than PE CVD silicon nitride films. Moreover, significant disadvantages occur when HDP CVD silicon nitride films are used in advanced ground-rule interconnect structures using low-k dielectric materials such as SiLK(trademark). The energetic reactions of the HDP process can enable interaction with and within the low-k dielectric materials causing undesirable changes to occur. These changes can be significantly mitigated by the use of PE CVD silicon nitride films. However, the poor performance of PE CVD films when compared to HDP CVD filmsxe2x80x94due to the poor adhesion of PE CVD films to copper surfacesxe2x80x94has heretofore precluded the integration of PE CVD films in advanced interconnect structures.
A method to improve the adhesion of PE CVD films is disclosed in U.S. Pat. No. 6,261,951 to Buchwalter et al., titled xe2x80x9cPlasma Treatment to Enhance Inorganic Dielectric Adhesion to Copper,xe2x80x9d the disclosure of which is incorporated herein by reference. In the Buchwalter et al. method, the copper surfaces of an interconnect structure are first exposed to a reducing plasma under conditions such that a new material layer is formed on the copper, wherein the new material layer comprises copper, silicon, oxygen and optionally at least one of carbon, hydrogen, nitrogen and fluorine. The exposure or xe2x80x9cpre-cleanxe2x80x9d step is carried out in a suitable reducing plasma such as hydrogen, nitrogen, ammonia and/or noble gases, at a temperature of about 20xc2x0 C. to about 600xc2x0 C., for a time of about 1 to about 3600 seconds. Moreover, the exposure step of the Buchwalter et al. method is conducted at a pressure of about 1 mTorr to about 20 mTorr, at a power of about 50 watts to about 10,000 watts, and at a gas flow rate of about 1 sccm to about 10,000 sccm.
The interconnect structure of Buchwalter et al. does not include a hardmask or polish stop layer. Moreover, Buchwalter et al. contemplate that their pre-clean method may be used in conjunction with various dielectric materials, and silicon dioxide is disclosed to be the preferred dielectric material. However, when the pre-clean method of Buchwalter et al. is used in conjunction with certain low-k dielectric materials such as SiLK(trademark), it has been discovered that the plasma exposure step may severely damage or destroy such low-k dielectric materials. Specifically, it has been observed that SiLK(trademark) dielectric material has been etched away during this pre-clean step at rates of several thousand angstroms per minute.
Thus, there is a need in the art for an advanced interconnect structure comprising low-k dielectric material and a diffusion cap layer formed by PE CVD, where the PE CVD cap layer is in strong adhesive contact with the metal conductor.
There is a further need in the art for an advanced interconnect structure comprising low-k dielectric material and a diffusion cap layer formed by PE CVD, where the low-k dielectric material is protected from damage during pre-clean of the metal conductor and deposition of the cap layer.
The problems described above are addressed through use of the present invention, which is directed to an interconnect structure formed on a substrate. In one embodiment, the structure comprises: a dielectric layer overlying the substrate, said dielectric layer being formed of a carbon-containing dielectric material having a dielectric constant of less than about 4; a continuous hardmask layer on said dielectric layer, said hardmask layer having a top surface; at least one conductor embedded in said dielectric layer and having a surface coplanar with the top surface of said hardmask layer; and a cap layer on said at least one conductor and on said hardmask layer, said cap layer having a bottom surface in strong adhesive contact with said conductor, wherein said cap layer is formed of silicon nitride by a plasma-enhanced chemical vapor deposition (PE CVD) process.
The present invention is also directed to a method of forming an interconnect structure on a substrate. In one embodiment, the method comprises the steps of: depositing a dielectric layer, said dielectric layer being formed of a carbon-containing dielectric material having a dielectric constant of less than about 4; depositing a hardmask layer on said dielectric layer, said hardmask layer having a top surface; forming an opening in said dielectric layer and said hardmask layer; filling said opening with a conductive material, thereby forming a conductor, said conductor having a surface coplanar with the top surface of said hardmask layer; exposing said conductor to a reducing plasma comprising at least one gas selected from the group consisting of H2, N2, NH3 and noble gases; and depositing silicon nitride on said conductor by a plasma-enhanced chemical vapor deposition (PE CVD) process, thereby forming a silicon nitride cap layer.